CARACAS (Reuters) - Venezuelan prosecutors accused a policeman on Monday of shooting dead a young female intelligence agent that he mistook for a kidnapper after her plainclothes team forced a suspected protester into their car.

The incident took place on Friday in a wealthy eastern part of Caracas, in an area where hooded opposition demonstrators clash nightly with riot police. At least 21 people have died in the country's worst unrest for a decade.

With various state security agencies operating in the capital's opposition-governed Chacao district, many residents had feared a confrontation between them and the separate police force that works for the municipality and its mayor.

In grainy security camera footage, members of national intelligence agency Sebin were seen grappling with a young man who was apparently carrying home groceries, before pushing him into the back of an unmarked car.

Chacao police says its officers on motorcycles pursued the Sebin vehicle onto a nearby highway, believing they were going to rescue a kidnap victim.

Interior Minister Miguel Rodriguez Torres says they caught up with the car in traffic and opened fire several times without warning, killing a 25-year-old female Sebin agent.

"There's no doubt they committed a homicide ... they broke all the rules of a police force and must go to prison," Torres told the Caracas-based television network Telesur.

"This was no kidnapping. It was a detention," he said, adding the 20-year-old man was captured by Sebin after being identified from videos of protesters engaged in "violent acts."

Venezuela's state prosecutor said in communique that one Chacao police officer was detained on suspicion of murder, while two others alleged to have been involved were given bail.

The country suffers from one of the worst crime rates in the world, and murders, robberies and kidnappings are common.

Residents in neighbourhoods where protesters have blocked roads and clash with police complain that the city's many gun-toting thugs are profiting from the chaos by committing crimes, including abductions, with even more impunity than normal.